The present invention relates to a magnetic actuator. The actuator may be used in any applications where a member, for example a diaphragm, piston or plunger mounted for reciprocating motion in a predetermined direction is to be actuated.
Magnetic actuators are known and operate by interaction between a magnetic field and electric current flowing in one or more coils or windings. Typically magnetic actuators include an electromagnet incorporating a fixed core and a winding associated with the core, influencing a movable armature also of soft ferromagnetic material. The armature is one or more permanent magnets mounted on a movable actuator member connected to the member to be actuated, the diaphragm of a diaphragm pump, for example, with the permanent magnets influenced by an electromagnet.
GB 1557453 shows a known moving-magnet actuator, which consists of a fixed soft magnetic E-core stator assembly and two parallely magnetised permanent magnets arranged so that they present opposite poles towards the stator. The magnets are attached to a pair of independent soft magnetic lever arms which are supported by a pivot point and attached to the compressor. A single phase coil is mounted on the core central limb, and when excited by an alternating current, the magnetic arms produce an alternating torque and hence displacement.
The pair of permanent magnets are arranged so the axes of magnetization are in opposition and the motion of the two arms are synchronous, however, this can produce excessive vibration.
The above systems have proved successful for a number of products but their achievable performance is limited by a number of factors and there remain several disadvantages in terms of their manufacture. For example, large air-gaps are required for mechanical clearance due to tolerance problems and allowance for wear of the pivot points, resulting in significant flux leakage and the creation of stray fields. Additionally the use of soft magnetic arms, although improving the magnetic circuit over non-magnetic arms by acting as back-iron for the magnets, introduces problems of significant leakage and stray fields along the arms due to the extension of the soft magnetic component from the magnet back to the pivot point. The interaction of these soft magnetic arms with the coil excitation field also produces a reluctance or saliency force which distorts the excitation force profile. Another inherent feature of such devices is the presence of unbalanced magnetic forces which act in a perpendicular direction to the desired direction of motion due to the attraction of the magnet and the swing arm component towards the soft magnetic stator assembly. These forces can lead to excessive wear on the pivot system, particularly due to the cyclic nature of the force when in operation.
With regard to the electromagnet design in these actuators, current designs are typified by stators of parallel tooth and slot designs which have a large pole area to produce the correct torque-displacement profile. In order to simplify lamination cross section and to allow simple coil location the pole widths may be extended along the entire tooth length. This leads to excessive volumes of material. The material is therefore under utilised due to low levels of flux. Additionally, this feature leads to relatively narrow slots, and in order to accommodate sufficient copper windings whilst also accounting for `creepage and clearance` the slots are typically deep and narrow, leading to slot leakage flux, i.e. flux produced by the coil which does not travel across the slot and is not available at the working air-gap for torque production.